1. Field of the Invention
The invention relates to a carbon steel as a texture-rolled strip steel, in particular for springs.
2. Description of the Related Art
Strip steel is frequently used, in technical areas of use, for the production of spring material, in particular also of springs that can be rolled up. Such springs are used, for example, in motor vehicle safety belts, as material for tape measures that can be rolled up, as cable roll-ups, or also as a roll-up element of dog leashes and in many other applications.
In the state of the art, the production of such roll-up springs is undertaken proceeding from the product of wire or strip, usually by means of conventional annealing of a strip steel on the basis of martensite, or by means of isothermal conversion to a fine-striped, pearlitic structure, with subsequent cold forming (so-called texture-rolled strip steel). For this purpose, unalloyed steels are typically used. In this connection, texture-rolled strip steel is understood to mean all strip steels and, in particular, spring strip steels, which have a clearly marked texture, i.e. a crystal orientation, in the final state. This crystal orientation brings about an improvement in the spring properties and a reduction in the fracture risk due to corrosion or mechanical damage crosswise to the crystal orientation. Usually, such a texture is produced by means of strong cold forming of the material by means of rolling, or also by means of drawing, without intermediate annealing.
The pre-rolled strip (for example having a thickness of 1.5 mm) is first austenitized (the carbon is brought into solution) at approximately 850° C., in an annealing system that operates continuously, then quenched in a lead bath at about 450 to 500° C., and held there until the isothermal conversion has completely taken place. The fine-lamellar pearlite structure that occurs in this connection is called sorbite, after the English inventor, or patenting structure. A good patenting structure has a lamellar distance of about 0.1 μm. The patenting strength is all the greater, the more fine-striped the sorbite and the less the distance between the cementite lamellae.
During the subsequent cold-rolling, which is predominantly carried out in reversing manner, significant deformations occur both in the ferrite lamellae and the cementite lamellae of the pearlite. In structure regions where the lamellae are disposed parallel to the rolling direction, the lamella distance is reduced as the result of the deformation. In contrast, in structure regions that lie perpendicular to the rolling direction, the lamellae are at first bent in wave shape, and, when greater shape changes occur, actually in hair-needle shape. A pure fiber structure is present as soon as all of the structure regions have disposed themselves to be parallel to the rolling direction, aside from the bending points.
This complicated and complex production process can be explained as follows in terms of metallurgy. During the deformation, the steel is subject to restrictions, since it must be deformed as a whole, without breaking down into individual grains. As a result, every grain must participate in the deformation, and every grain must coordinate this deformation with its adjacent grain, in order to allow cohesion along their grain boundaries. However, the grains of the strip steel have different orientations. If these have an external stress applied to them as the result of the rolling process, then those grains that have advantageously oriented slide systems, in other words a higher Schmid factor (the conversion factor between externally applied tensile and pressure stress σ and the shear stress τ that acts in the slide plane), will already deform while the critical shear stress has not yet been reached in other, less advantageously oriented grains. The deformation of an individual grain therefore leads to a shape change that is not shared by the surroundings, which do not deform plastically. The shape change is elastically suppressed, which can lead to high internal stresses, and finally, as a result, the critical shear stress is also reached in the adjacent grains. Only when all the grains of the strip steel deform plastically is the stretching limit of the material reached.
In addition to the mechanism indicated above, which increases strength, by means of cold rolling, in addition the increase in strength by means of so-called mixed crystal hardening can be used. This is a consequence of the interaction of the alloy atoms with the offsets that leads to hindrance of the interaction. The effect of the foreign atoms can take place in three different ways, in this connection:                Paraelastic interaction: This lattice parameter effect is brought about by means of the different atom size of the foreign atoms, in comparison with the matrix atoms; the installation of the foreign atoms into the crystal lattice causes stresses.        Dielastic interaction: The interaction of this shear modulus effect is based on the fact that the energy of an offset is proportional to the shear modulus G.        Chemical interaction: This mechanism, also called Suzuki effect, is based on the fact that the energy of the stacking defects depends on the composition.        
Aside from the structure-technology influence variables, the properties of the thin-walled rolled steel that is relevant here are also decisively influenced by the surface topography. A geometrically ideal, i.e. completely smooth surface cannot be achieved using conventional technical means; instead, a technical surface with defined superimpositions of individual shape deviations (scallop heights and wave depths) is produced. Cold rolling is a bound deformation (flat shape change state). In this connection, the unstraightened surface of the descaled, pickled hot strip is straightened to form a textured strip, with an increasing rolling pass number, as a result of the relative movement between roller and rolled material.
The rolling passes on the special frames flatten the surface of the steel. The decrease in roughness amounts to between 60 and 90%, as a function of the product. It depends on the initial roughness, the roller roughness, as well as on the shape change resistance of the material. With an increasing number of rolling passes, the roughness of the surface approaches a limit value. Variation parameters for setting the required topography are, among others, roller gap geometry, pressure distribution, roller speed, and strip tension. The working roller as the deformation tool exerts a decisive influence on the strip surface, in this connection. Its surface profile changes with the roller travel. The grinding contour of a freshly installed roller is speedily worked off; flattening is particularly intensive at the beginning, and then asymptotically reaches a certain limit value.
With spring steels produced with the observance of the aforementioned production parameters, it is possible to achieve excellent spring properties even at very high stress cycle numbers. These materials are therefore widespread for the special applications described.
However, it has turned out that in the case of spring materials for roll-up springs having only a very slight strip thickness in the most-stressed range, for example in the production of roll-up springs for dog leashes in the thickness spectrum of typically 0.10-0.19 mm strip thickness, the structure components that form in the structure that occurs are not allowed to be larger than about 20 μm and thus smaller than about ⅕ of the strip thickness of the spring steel strip. In the case of larger components or inclusions, a notch effect can otherwise occur at the inclusions or structure components, and this can lead to destruction of the strip steel. Guaranteeing such a fine-grained structure is problematic in connection with the conventional production of corresponding spring materials described above, at the required sheet-metal thicknesses.